linux 常用函数在线文档
http://www.kernel.org/doc/man-pages/
http://man7.org/linux/man-pages/dir_all_alphabetic.html
参考：
https://my.oschina.net/eechen/blog/206443
http://www.kernel.org/doc/man-pages/
http://man7.org/linux/man-pages/dir_all_alphabetic.html
参考：
https://my.oschina.net/eechen/blog/206443